Changes of state: Changes of state are physical changes which differ from chemical changes because the material recovers its original properties if the change is reversed.

The idea

A change of state is physical: the particles keep their identity, only their arrangement and energy change. If the change is reversed, the material gets back its original properties. Steam condensed to water can be frozen to ice. The ice melts at 0 °C, and the resulting water boils at 100 °C, having the same density and appearance as before. In a chemical change, new substances form with different properties. For example, burning magnesium gives white magnesium oxide, which cannot be turned back into shiny magnesium by simply cooling. Comparing properties before and after, and testing whether reversing restores them, distinguishes physical from chemical change.

What earns the marks

• Define a physical change as one where no new substance forms and the particles keep their identity.
• State that reversing a change of state restores the original properties, such as melting point, boiling point, density and appearance.
• Contrast with a chemical change, where new substances with different properties form and simple reversal does not recover the original material.
• Use a concrete example, such as ice → water → steam → water → ice, and note that the final ice behaves exactly like the starting ice.
• Explain that particle arrangement and energy change during a change of state, but the particles themselves are unchanged.

Common mistakes and how to put them right

• Calling melting or boiling a chemical change because it looks dramatic.

  Put it right: No new substance forms, so it is physical.

• Claiming that reversing a chemical change is impossible.

  Put it right: Some chemical changes are reversible. While many require chemical reagents to reverse, some, like the decomposition of ammonium chloride, reverse simply by cooling.

• Saying the material 'recovers its original properties' without naming any property.

  Put it right: Give a specific property, such as melting point, density or colour.

Exam tips

• When asked to compare, give one feature of a physical change and one feature of a chemical change, then link each to an example.
• Use the word 'reversible' only when you can state the reverse process and the property recovered.
• If a question gives data, compare a numerical property before and after, such as melting point or density, to support your answer.